Black Outline Around Material Node

As detailed in the screencap below, the red Material there on Robin’s costume has a green Material overlaid on it via Material Nodes. This green Material is located based upon the Factor value of a UV Map.

I would like to have a matte black color protruding from underneath the green, so that there’s a black line where the green meets the red (this is to represent an artist’s pencil stroke). Any solutions would be greatly appreciated.

(Please note that I’d like a solution with Material Nodes, rather than doing all new UV Maps, because there are many other Materials on different characters with the same circumstances.)

Thanks as always.